EVALUATING THE ROLE OF IRON IN ANTIBACTERIAL MINERALS

Antibiotic resistant bacteria present a challenging human health problem requiring new antibacterial agents that can effectively kill resistant bacteria while promoting skin cell growth. Research has shown that certain clay mineral deposits have the ability to prevent the growth of a broad spectrum of bacteria, including antibiotic resistant strains. The host rock in these clay deposits consists of porphyry andesite that has been hydrothermally altered into mixed layered illite-smectite, pyrite and quartz. The leachates (aqueous solutions equilibrated clays for 24 hrs) from these clays are also antibacterial, containing ferrous iron from pyrite or adsorbed on the interlayers of 2:1 expandable clay minerals. Dissolved ferrous iron can form hydroxyl radicals and hydrogen peroxide, which can degrade bacterial lipids, proteins and DNA.

It was proposed that nano-iron sulfides mixed with clay minerals release soluble iron or other transition metals at low pH, which are effective in killing antibiotic resistant bacteria due to the generation of reactive oxygen species (ROS) and damage of bacterial membranes. To test this, model gram-negative and gram-positive bacteria (Escherichia Coli ATCC 25922 and Staphylococcus Epidermidis ATCC 14990) were reacted with antibacterial minerals (200mg/400µl) and aqueous leachates at 37˚C. Antibacterial susceptibility testing reveals that low pH (2.8-3.5) leachate samples containing mM levels of soluble Fe, Al and Ca are bactericidal. Transmission electron microscopy (TEM), Scanning transmission X-ray microscopy (STXM) and TBARS (thiobarbituric acid reactive substances) assays were used to assess the antibacterial mechanism.

TEM imaging of bacterial cells reveals cytoplasmic condensation and intracellular voids with the occasional accumulation of 20-100nm particles on the cell walls of both gram-positive and negative cells. STXM iron maps of single bacterial cells reveal that soluble iron is adsorbing to the cell walls of the bacteria, with Fe(II) > Fe(III). The TBARS assay indicates that adsorption of iron onto the bacterial cell wall results in lipid peroxidation and the release of mono-aldehydes. Results from this research may reveal new antibacterial mechanisms involving natural minerals that have traditionally been overlooked by the medical community.